In the field of magnetic recording media, there is a strong demand for high-density recording. Surface smoothness of the magnetic layers is one of the known measures in coping with such demand.
However, the smoothness of the magnetic layer surface causes the coefficient of friction between the running magnetic layer of the magnetic recording medium and the recording or reproducing device's parts in contact with the magnetic layer to be so heightened that the magnetic layer is apt to suffer damage or peel off in a relatively short period of use.
For overcoming the above problem, lubricating agents such as fatty acids, fatty acid esters, hydrocarbons, and silicone compounds have so far been used.
With the recent spread and popularization of flexible disk drives for home use such as those for VTR's, personal computers, or word processors, the conditions under which magnetic recording media are used have become varied in wide ranges including low-temperature conditions and high-temperature and high-humidity conditions. Therefore, the running durability of the magnetic recording media should be stable under various expected conditions. The above-mentioned known lubricating agents, however, are insufficient for such purpose.
As another measure to improve running durability, incorporation of abrasive agents (hard particles) in magnetic layers has been proposed and is being practiced. However, this technique is disadvantageous in that in order to minimize the improvement in running durability of magnetic layers, an abrasive agent should be incorporated in a considerably large amount. In other words, it is difficult after all to obtain good running durability without impairing electromagnetic characteristics and head abrasion resistance.
Use of organofluorine compounds as lubricating agents is conventionally known as disclosed in, for example, JP-A-58-164023, JP-A-59-33624, JP-A-57-12417, JP-A-57-37730, JP-A-59-152525, and JP-A-62-1116. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".)
Although magnetic head staining can be diminished by these organofluorine compounds, the dispersing effect of such lubricating agents is so insufficient that the resulting magnetic layers cannot have satisfactory smoothness. It has further been proposed in JP-A-59-167841, for example, to incorporate an organofluorine compound, as a lubricating agent, having in its molecule a perfluoroalkyl group and a poly(alkylene oxide) group, into a magnetic coating composition containing a dialkyl sulfosuccinate as a dispersing agent, thereby to improve the durability and heat resistance of magnetic layers without impairing the ability to disperse magnetic powders.
However, since the above fluorine compound shows good compatibility with binders, it is difficult to balance the amount of the fluorine compound to be incorporated with the amount thereof which will migrate to the magnetic layer surface. Thus, the above-proposed fluorine compound has been unable to produce a stable effect when incorporated in a small amount.